Skier Balance posts

In Part 3 of the mechanics of platform angle I suggested that some unidentified force or forces are at work that enable elite skiers to alter the angle of attack of the applied force R so that it is more aggressive in terms of cutting (carving) a step into the surface of the snow. I asked the reader what the components of the applied and reaction forces would look like.

One reader correctly identified two separate forces acting on the transverse plane of the platform of the outside ski; one oriented vertically at 90 degrees to the plane and a second force oriented parallel or 180 degrees to the transverse plane with the vector aligned into the snow.

The right hand graphic below shows the 90 and 180 degree components of the angular force acting on the platform in the left hand graphic.

The right hand graphic below is the same as the graphic above but with the angular force superimposed over the 90 and 180 degree components.

I am taking the discussion of platform mechanics in small steps in order to provide the reader with a chance to assimilate the issues and ask questions if my discussion is not clear.

THE SHOCKING TRUTH ABOUT POWER STRAPS AS A REFERENCE

Most of the views of the series on the Mechanics of Platform Angle are accompanied by views of The Shocking Truth About Power Straps which contains quotes from the medical textbook The Shoe in Sport (published in German in 1977 as Der Schu im Sport). This medical textbook has been invaluable to my efforts.

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions.

These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.

The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).

It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

– Biomechanical Considerations of the Ski Boot (Alpine)

The question for this post is what is the source of the 180 degree force? Please consider Dr. E. Stussi’s comments above when contemplating an answer to this question.

For the sake of simplicity I have started the discussion of the mechanics of platform angle with opposing static forces acting across the platform edge/snow surface (i.e. ground) interface. The use of static forces and drills to illustrate platform mechanics is not realistic because skiing involves the acceleration and deceleration of a body (i.e. mass). A realistic discussion must consider all significant external and internal forces and the effects of momentum and inertia. A key component of any discussion of this nature is the orientation of the platform or transverse base angle of the outside ski in relation to the vector of opposing applied and reaction forces and the angle of the vector with the plane of the surface of the snow. The mechanism of control of the platform angle must also be considered.

The objective of the initial posts on the mechanics of platform angle is to create a set basic principles to serve as a frame of reference for multi disciplinary dialogs on the mechanics, neurobiomechanics and physics of skiing.

In my last post I discussed how the shear or slip component of an applied angular force acting on a surface or body will increase in magnitude as the angle of attack decreases and becomes more aligned with the plane of the surface while the normal component of the applied and force will decrease in magnitude. As this happens the tendency of the force applied to the snow that would cause it to penetrate into the surface and cut a step will decrease. As the platform angle with the snow becomes increasingly more perpendicular and the vector of the applied force becomes more aligned with the plane of the surface of the snow the component of shear force will increase and the ski will slip regardless of a perpendicular orientation of the platform with the applied force R.

Platform Forces: A different perspective

The force diagram below shows how the angle of the point of application of force applied to the inside edge of the platform that would cause it to cut a step into the surface of the snow becomes progressively less aggressive as the vector of the opposing forces becomes more aligned with the plane of the surface of the snow.The graphic below shows another way looking at angular forces acting on a surface. This graphic only shows the components of the applied and reaction forces. The advantage of showing the components is the magnitude of the normal and shear or slipping forces can be shown in relation to each other. I’ve taken some liberties in showing the normal GRF force as having greater potential magnitude than any force applied by the platform of the ski.

As the angle of the platform with the surface of the snow increases (becomes closer to perpendicular) the magnitude of the normal force will decrease. As it does the magnitude of the shear (slipping) force will increase in lock step. As the magnitude of the shear (slipping) force increases, the potential magnitude of the GRF shear component will decrease and the platform will tend to slip and not cut a step into the surface of the snow.

Since we know that elite skiers and racers can carve a step or ledge into the surface of very hard pistes at high platform angles it is reasonable to assume that some unidentified force or forces are at work that are altering the angle of attack of the applied force R so that it is more aggressive in terms of carving a step into the surface of the snow as shown in the graphic below. What would the components of the applied and reaction forces look like?As always, comments, suggestions and objective criticisms are welcome. In Part 4 we will look for the elusive forces that make skis carve at high platform angles.

I believe the single most important factor affecting a skiers’ balance and directional control of a ski is the ability of the balance system to effect dynamic balance of the angle of the platform of an edged ski with the surface of the snow . So I am going to focus my efforts on explaining the mechanism of dynamic balance.

Skiing is an interaction of the skier with the snow. Since the interface of the interaction is the inside edge of the ski and the snow I’ll start here with an explanation of the principles of the associated mechanics.

Snow = Ground

Snow is an extension of ground. Hence we speak in terms of ground reaction force or GRF. In moving over the snow a skier is interacting through a layer of snow with the ground. In the name of consistency firm pistes will be the reference for the discussion of forces.

The interface with the ground is the base plane of a ski in particular the juncture of the transverse base and sidewall planes. In this interaction the angle of the base plane with the surface plane of the snow is the plane of balance for the skier in terms of the management of angular forces.

Edging Forces are not Normal

In order for the platform of a ski to hold and not slip under the weight of a skier the edge and the adjacent portion of the base must cut a step or ledge into the surface of the snow. But the portion of the ski that must cut a step into the surface of the snow is not a knife edge. It is more like a knife blade on the flat with the blade aligned perpendicular to the angle of attack of the force applied by the skier.

Typical force diagrams in technical discussions of skiing only show opposing angular forces with a platform perpendicular to the vector of the forces. The opposing angular forces R and GRF are said to cause the platform of the ski to cut a step or ledge into the surface of the snow as shown in the graphic below. One problem is that sketches such as the one below don’t show the perpendicular (Normal) or horizontal (Shear or Slip) components of the opposing angular forces R and GRF.

When an applied force is Normal to a surface (perpendicular) the penetration or cutting action is maximal. But when a force applied to a surface is less than perpendicular it will have components of Normal and Shear or Slip forces such as shown in the graphic below. At an angle of attack of 45° the Normal and Shear components of the applied and GRF forces will be equal. But as the angle of attack decreases and becomes more aligned with the plane of the surface the Shear or slip components will increase in magnitude and the Normal components of the applied and GRF forces will decrease in magnitude.

As this happens the tendency of an applied force acting on a body like the platform of a ski that would cause it to penetrate into the surface of the snow and cut a step will decrease as the angle of the platform with the snow increases. As the platform angle increases so will the tendency of the ski to slip and not hold an edge. The components of opposing applied and GRF forces acting on a solid body or surface are determined mathematically by sine/cosine. They are not negotiable. Nor can their impact on ski platform mechanics be ignored.

In my next post I will discuss the real force that makes the platform of a ski cut a step or groove into the surface of the snow that the edge of the ski tracks in.

In order to engage in an interactive productive dialog on issues pertaining to ski technique and related equipment a frame of reference based on validated, non-negotiable principles of physics, mechanics and (neuro)biomechanics as well as a schedule of defined reference terms such as exists in the sciences of mechanics, anatomy and physics is essential. Defined technical reference terms help ensure all participants in a discussion are on the same page.

I decided to start the new direction of The Skier’s Manifesto with a critical examination of the mechanics of platform angle starting with a schedule of the technical terms associated with platform angle and their definitions. Additional technical terms and their definitions will added in future posts according to the content of the discussion. The intent at this point is to start with a basic discussion of forces applied to a rigid body and/or surface (in this case, the surface of the snow) and then expand the scope of the discussion in future posts. Agreement on terms and definitions is important. So please comment if you feel one or more the following terms are inappropriate or inaccurate or should be expanded and/or refined.

Technical Terms associated with Platform Angle

Platform Angle: the angle of the transverse aspect of the body of the ski underfoot with the surface of the snow.

Edge Angle: the angle of the edge of the ski in relation to the plane of the transverse aspect of the body of the ski adjacent the edge.

Force: an unopposed interaction that will change the motion of an object. A force has both magnitude and direction, making it a vector quantity.

Force Vector: the magnitude and direction of a force.

Applied Force: a force applied to a rigid body or surface.

Reaction Force: a force that opposes a force applied to a rigid body or surface.

Normal Force: a force acting perpendicular to a rigid body or surface that is resisting a force applied to it.

Angular Force: a force applied to a rigid body or surface that is not normal (perpendicular) to the rigid body or surface to which the force is applied.

Angle of Attack: the angle an angular force forms with the rigid body or surface to which it is applied to.

Resultant Force: also known as Net Force, is a single force associated with torque obtained by combining a system of forces and torques acting on a rigid body.

Technical discussions of the forces associated with the angle of the platform with the snow typically show opposing resultant and ground reaction forces implying a state of balance of the forces acting on platform created by the outside ski underfoot.

Schematic diagrams showing forces acting on the platform created by the body of the ski underfoot often show two opposing forces in alignment with each other acting close to or at the axis point created by the inside edge of the outside ski. Or diagrams may simply show opposing forces aligned with each other implying the existence of a state of equilibrium.

In my next post I will discuss whether the above force diagrams accurately reflect a state of equilibrium of the forces acting on the platform of the outside ski. Please join the conversation.

This post was originally published on October 23, 2016. I have revised the post to clarify that the SR Stance applies to the load phase of a turn that occurs in what is commonly referred to as the bottom of a turn and that the joint angles of the SR Stance are configured by the major muscles in isometric contraction. When external forces cause the muscles to lengthen or stretch this will trigger the myotatic or stretch reflex. Because the myotactic reflex is a spinal reflex it is activated in 1 to 2 thousandths of a second. As such, it is both rapid and powerful.

The SR Stance configures some of the most powerful muscles in the body in a state of isometric contraction so that the powerful myotactic stretch reflex can maintain the angles of the ankle, knee, and hip and keep the CoM of a skier in balance on their outside ski in the most powerful position in the load phase of a turn.

The SR Stance is best learned outside the ski boot in an environment where the feet and legs are free from any influences. One of the benefits of learning an SR Stance outside the ski boot is that, once learned, it provides a reference against which to assess whether a ski boot supports the functional parameters of the skier. If it doesn’t, the SR Stance can be used as a reference to guide equipment modification and establish when and if it meets the functional requirements of the skier.

The SR Stance tensions the pelvis from below and above; below from the balls of the feet through the PA-soleus-gastrocnemius-hamstring muscles to the pelvis and above from the shoulders-latissimus dorsi-trapezius muscles to the pelvis.

The graphic below shows the Achilles Tendon junction with the PA at the heel bone.

The graphic below shows the 3 major muscles of the leg associated with the SR stance.

The Soleus (left image in the above graphic) extends from the back of the heel bone (see previous graphic) to a point just below the knee. It acts in concentric contraction (shortening) to extend or plantarflex the ankle. In EC-SR, the Soleus is under tension in stretch in isometric contraction.

The Soleus is one two muscles that make up the Triceps Surae.

The Gastrocnemius (center image in the above graphic) extends from the back of the heel bone to a point just above the knee. It acts in concentric contraction (shortening) to flex the knee. In EC-SR, it is under tension in isometric contraction to oppose extension of the knee.

The Hamstrings (right image in the black rectangle in the above graphic) extends from a point just below the knee to the pelvic girdle. It acts in concentric contraction (shortening) to flex the knee. In EC-SR, it is under tension in isometric contraction to oppose extension of the knee.

A number of smaller muscles associated with the SR that will be discussed in future posts.

The graphic below depicts the 3 steps to learning an SR Stance.

The first step is to set up a static preload on the shank (shin) of the leg by tensioning the soleus muscle to the point where it goes into isometric contraction and arrests ankle dorsiflexion.

The static preload occurs when the tension in the soleus muscle of the leg simultaneously peaks with the tension in the sheet-like ligament called the plantar aponeurosis (PA). The PA supports the vault of the arch of the foot. The soleus is an extension of the PA. This was discussed in my post ZEPPA-DELTA ANGLE AND THE STRETCH REFLEX.

While barefoot, stand erect on a hard, flat, level surface as shown in the left hand figure in the graphics above and below. The weight should be felt more under the heels than under the forefoot.

Relax the major muscles in the back of the legs (mainly the hamstrings) and allow the hips to drop and the knees to move forward as shown in the right hand figure in the graphics above (1.) and below.

As the knees move forward and the hips drop towards the floor the ankle joint will dorsiflex and the angle the shank forms with the floor and the angle of the knee, will both increase until a point is reached where the shank stops moving forward on its own. Movement of the shank will probably be arrested at a point where a plumb line extending downward from the knee cap ends up slightly ahead of the foot. This is the static preload shank angle. It is the point where the soleus and quadriceps muscles go into isometric contraction.

2. From the static preload shank angle, while keeping the spine straight, bend forward slightly at the waist. The angles of the shank (ankles) and knees will decrease as the pelvis moves up and back and the CoM moves forward towards the balls of the feet. This will cause the muscles of the thigh to shift from the Quadriceps to the Hamstrings. Bending at the waist tilts the pelvis forward. As the pelvis tilts forward, it tensions the Hamstrings and Gastrocnemius causing the knee and ankle to extend to a point where extension is arrested by the muscles going into isometric contraction. Tension in the Hamstrings and Gastrocnemius extends the lever arm acting to compress the vault of the arches of the feet from the top of the shank to the pelvis thus increasing the pressure on the balls of the feet through Achilles-PA load transfer.

3. From the position in 2., round the back and shoulders as you bend forward from the waist.

Make sure the core is activated and tightened as you round the back and shoulders. Pull the shoulders forward and towards each other as the back is rounded so as to form a bow with the shoulder girdle. Looking down from above, the arms should look like they are hugging a large barrel.

Repeat steps 1 through 3. Pay close attention to the changes in the sensations in your body as you work through each step. If you bounce up and down lightly in the position in Step 3., the angles of the joints in your stance should return to the static preload position between bounces.

With the ski boot and Zeppa-Delta ramp angles configured to enable an SR stance, your ski boots will work for you and with you instead of the other way around.

In my next post, I will go into greater detail on how rounding the shoulders and holding the arms in the correct position optimally activates the muscles associated with the SR stance.

I found the wild card result in the skate tests discussed in my last post shocking but not unexpected. I had known for decades that ski boots can dramatically impact user performance. But until the skate tests I had no way of confirming my subjective observations, which could be summarily dismissed as nothing more than my opinion. The results of the skate test provided convincing support for my long held assertion that testing the effect of ski boots on the user with a set of realistic performance metrics is absolutely essential.

In the graph below of Peak Force all 5 competitive skaters improved in the NS.

Skater number four went from the skater with lowest Peak Force to the skater with the highest Peak Force. But skater number one, who had the fourth highest Peak Force in their OS, hardly saw any improvement in the NS whereas skater number four realized over a 100% increase in Peak Force!But the real shocker was in Impulse Force. As expected, results varied. But the Impulse Force of skater number one actually decreased slightly in the NS!Without a standardized, validated test protocol there is no way of knowing how their ski boots affected the performance of the competitors in the Soelden GS or any race for that matter. Guessing should not be acceptable.

Challenging course conditions, especially in GS, are the litmus test of dynamic stability. The 2018 World Cup GS at Soelden had challenging conditions in spades.

The ability to rapidly achieve dynamic stability across the inside edge of the outside ski is key to moving the Center of Force forward to the point where the biokinetic chain of the outside leg attains sufficient tension to enable the stretch reflex. The stretch reflex (SR) can then modulate pertubations due to asperities in snow surface and terrain with ankle strategies. The principle muscle in ankle balance synergies is the soleus. Dynamic stability enables a racer to float between turns, accelerate under gravity then land on line and load the outside ski. A racer with good dynamic stability is on and off the edges in milliseconds and back into the float phase. Like a skilled gymnast elite skiers and racers can choose their line and stick their landing. Tessa Worely excelled at this in the 2018 Soelden GS.

Tell Tale Signs of Dynamic Stability

Key indicators of dynamic stability are a quiet upper body and the speed at which a racer achieves their line and crosses over into the new turn with their upper body. It’s like watching a flat rock thrown low skipping off water; fly-skip-fly-skip.

In my post, WHY YOUNG TALENTED SKI RACERS FAIL AND EVENTUALLY QUIT RACING (1.), I discuss the 3 levels of balance:

The first reaction is the myotatic stretch reflex, which appears in response to changes in the position of the ankle joints, and is recorded in the triceps surae muscles. This is the earliest mechanism, which increases the activity of the muscles surrounding a joint that is subject to destabilization. Spinal reflex triggered by the myotatic stretch reflex response causes the muscle to contract resulting in the stiffening of the surrounding joints as a response to the stimulus that has disturbed the balance. For example, changes in the angle of the joints of the lower limbs are followed by a reflexive (fascial) tensioning of adjacent muscles. The subsequent release of the reaction prevents excessive mobility of the joints and stabilises the posture once again.

The next reflex in the process of balancing is the balance-correcting response, which is evoked in response to a strongly destabilising stimulus. This reactive response has a multi-muscle range, and occurs almost simultaneously in the muscles of the lower limbs, torso and neck, while the mechanisms that initiate the reaction are centrally coordinated.

The last of the three types of muscular reaction is the balance-stabilising response. In a situation of a sudden loss of balance, a myotatic stretch reflex first occurs and is then is followed by a balance correcting response, which prevents or attempts to prevent a fall.

If a racer is no able to use the myotatic reflex (Green = Normal) balance response, the CNS shifts to Level 2 (Orange = Caution) or even Level 3 (Red = DANGER).

Level 1 balance is characterized by a stable, well-controlled upper body (aka quiet upper body) with well controlled and directed positions of the arms.

When the myotatic (stretch) reflex is compromised by restriction of the ankle flexion range required to tension the soleus the balance system will shift to level 2 or level 3 depending on the degree of interference. As the degree of interference with required range of ankle flexion increases the degree of reflexive balance will progress from small, rapid, reactive arm movements to gross reactive arm movements that eventually include gross movements of the torso.

The authors of the Polish skier balance study cited in my post state that ski boots exclude the ankle joint complex from the process of maintaining the stability of the body. However, I don’t believe this is the case with all skiers and especially all racers as evidenced by Soelden video of Tessa Worley, Federica Brignone and Michaela Shiffrin. In my next post I will discuss what I look for in analyzing that suggests dynamic stability and especially a lack of dynamic stability and the indications of compromise and the potential cause.

In the meantime, here’s something to think about.

Early in my boot modification career I came to the conclusion that some skiers, especially racers, were born with the right shape of feet and legs (2.) and this explained why they could ski in ski boots right out of the box with minimal or no modifications better than the majority of skiers even after extensive boot modifications. In a recent series of posts I discussed the results of the 2012 skate study that I modified hockey skates for; the NS (New Skates – Blue bars in the graphics below). The modifications I made were based on ski boot modifications that had resulted in dramatic improvement in performance and race results. Although I optimistically predicted improvements in performance metrics of at least 10% (110%) based on my experience with World Cup skiers, I knew that there was the possibility of a wild card competitive skater who was already close to their maximum performance in their OS (Own Skates – Red bars in the graphics below). If this were the case the skater would realize minimal improvement from the New Skates.

My previous posts only included the results for four competitive skaters. There were actually five competitive skaters in the study. Skater number 1 was the wild card. Look what happened to the results when the wild card skater was added.Look carefully at the graph of the Impulse Force below. Compare Skater number one’s Impulse Force results with the Peak Force results in the preceding graph.This raises the question: Do Tessa Worely, Federica Brignone, Mikaela Shiffrin and other top World Cup racers have the right shape of feet and legs or do they have the right modifications made to their ski boots.